The invention relates to an image projection module comprising, in this order, an illumination system including a radiation source and an image display system having at least one display panel for modulating an illumination beam to be supplied by the illumination system with image information, the illumination system comprising an integrator system having an exit surface which is situated in an exit plane.
The invention also relates to an image projection device provided with such an image projection module.
An image projection module of the type described in the opening paragraph is known from, inter alia, U.S. Pat. No. 5,098,184. The image projection module described in this Patent comprises an illumination system for supplying an illumination beam and an image display system having at least one display panel for modulating this light beam in conformity with image information to be protected. In addition to a radiation source, a reflector behind the radiation source and a condenser lens, the known illumination system also comprises an integrator system. A further lens ensuring that all re-images are superimposed in the plane of the display panel is arranged behind the integrator system.
The display panel may be, for example, a liquid crystalline display panel. Such a panel comprises two optically transparent plates enclosing a layer of liquid crystalline material and provided with a two-dimensional array of pixels. Each pixel comprises an active picture element which shuts off the relevant pixel from the exterior when it is not addressed. The active picture elements acquire their signals via row and column electrodes which also form part of the pixels. Moreover, there are small areas within the pixels which cannot be driven, which is due to photolithographic limitations. The pixel portions which are occupied by the row and column electrodes and the areas which cannot be driven will hereinafter be referred to as passive portions. The ratio between the active portion, which actually switches, and the passive portion defines the geometrical aperture of the display panel.
However, a liquid crystal display panel as described above causes a number of drawbacks as regards the light output of the image projection module.
In current LCD projection systems, the aim is, inter alia, miniaturization. However, if the display panels are reduced in size, while maintaining the resolution, the geometrical aperture will decrease so that considerable light losses may occur and the light output of the system will decrease. A known solution to this end is the use of a microlens array at the illumination side of the display panel, ensuring that the incident light is concentrated on the active pixel portions. In the case of miniaturization, the microlens arrays should also be reduced, as well as the distance between the microlenses and the liquid crystalline layer. These requirements relatively complicate their manufacture.
Moreover, the liquid crystalline display panel is generally situated between two polarizers if the display panel adds image information to the light beam by means of polarization modulation. If unpolarized light is incident on the display panel, substantially half of it will be absorbed by the first polarizer, which gives rise to heating of the polarizer and the display panel so that the display panel may be damaged. A known solution is to prepolarize the light in the light path between the radiation source and the display panel. However, this requires an extra component such as, for example, a polarizing beam splitter.
Moreover, if a color image projection device with a single display panel is concerned, approximately 2/3 of the light incident on the display panel will be lost. In fact, a color display panel creates a colored dot consisting of three separate pixels which cannot be distinguished with the naked eye. Within a set of three pixels, each pixel comprises a different color filter for absorbing two of the three primary colors which, in their turn, will be passed by one of the two other color filters. Each pixel thus passes only 1/3 of the incident light. In known image projection devices, this phenomenon is solved, for example, by splitting up the light beam of the radiation source into three monochrome sub-beams such as, for example, angular color separation, as described in U.S. Pat. No. 5,161,042.
In short, since light is incident on the passive portions of the display panel and since light having the unwanted direction of polarization and/or the unwanted wavelength is incident on the active portions, a considerable quantity of the light supplied by the radiation source is lost which thus cannot contribute to the formation of the image. To prevent this, a number of possibilities were presented in the past. A drawback is, however, the complexity of the image projection device realized in these manners. With further miniaturization, this complexity even increases, because the requirements for positioning and manufacturing the different components thereby become more stringent.